I was one of the big initial proponents of Group Calendar for Redeemed (about a year ago), and am the first to acknowledge that it was a failed experiment.
1) For whatever reason, individual setups were a nightmare ... version issues, the thing wouldn't sync, etc etc. It was a big distraction trying to get people up and running.
2) The performance hit is an issue ... while is seemed to impact people differently, frankly introducing any lag into a 25-man raid because of a calendar mod just isn't going to be accepted.
3) As mentioned above, without critical mass it just doesn't work well. It's like the /lfg functionality within WoW ... it's great in theory but there just aren't enough participants for the matchmaking to work.
4) Back a year ago, for those of us who got through #1-#3 ok, the problem was that even for a scheduled event, you'd get no-shows. Lots of 'em. Reality is that for Redeemed, many many of us have RL priorities that >> WoW. While there's some probably that if we've, say, signed up for a raid and need to cancel that we'll be able to pop a quick post to the forums...Group Calendar doesn't let you do that (you can look at schedules extra-game but not change them). So, again, the matchmaking didn't work out in practice even when it seemed ok in theory. Again, given our schedules, you're better off getting in game and lfg'ing it in guild chat and just having the mindset that sometimes it'll come together and sometimes it won't.
5) Also, reality is that php is out best solution for trying to arrange the 25-mans raids and those are really the ones where the logistical complexity is most needing of a more formal mechanism. It works well enough right now and certainly ain't broke so we ain't gonna mess things up by trying to fix it. One of the reason why php and the forums work well, I think, is that it is outside the game and something that people can check and modify and update from work or whatever.
Anyhow, as most have mentioned, GroupCalendar seems like a really cool mod and I don't think that anyone's saying that you should't install it ... but I think it's pretty definite that we won't make any efforts to make it a guild standard mod like Omen/Thread and Bigwigs/Deadly are.